1. Field of the Invention
The present invention relates to an optical coupling module for optically coupling an optical fiber with an optical waveguide, and a method for fabricating the optical coupling module. More particularly, the present invention relates to an optical coupling module, in which a substrate is anisotropically etched to form thereon two self-aligned grooves having different widths, and an optical fiber is then disposed in one of the grooves while an optical waveguide is disposed in the other of the grooves, so that the optical fiber and optical waveguide can be self-aligned with each other, and a method for fabricating the optical coupling module.
2. Description of the Prior Art
Recently, the amount of information transmission through the internet and data transmission between respective boards of PCs have been rapidly increased. In order to cope with this tendency, there is a increasing demand on optical communication for allowing broadband communication in place of conventional copper wire communication. In such an optical communication system, the optical coupling efficiency at optical coupling portions, such as between optical fiber and optical fiber, between light emitting element and optical fiber, between optical fiber and light receiving circuit, or between light emitting element to light receiving circuit, becomes a significant factor for communication quality in view of efficient transmission of signals. Especially, since an optical coupling circuit with a single-mode optical fiber as media or a conventional optical circuit should have a permissible alignment error of 1 to 2 μm, a simple and precise assembly technique is required.
Among conventional optical coupling methods, an active alignment method and a passive alignment method have been generally used. According to the active alignment method, in a state where a light emitting element is driven or an optical signal is applied to an optical fiber, the light emitting element or other optical circuit elements (for example, optical fiber, optical waveguide and light receiving circuit) should be moved in a vertical or horizontal direction until a position where the optical coupling efficiency reaches the maximum value is determined. A relative position corresponding to the determined position is then fixed with an adhesive or the like. However, although a constant coupling efficiency can be obtained by the active alignment method, the light emitting element should be driven throughout the assembly process. In addition, it takes much time to align the light emitting element to achieve the optimal optical coupling efficiency. Thus, there is a disadvantage in reducing the manufacturing costs.
According to the passive alignment method, desired assembly precision can be obtained by mechanically positioning an optical fiber and other optical circuit elements on a precisely manufactured jig. Since the size or distribution of manufacturing errors has not yet been sufficiently reduced, there is a disadvantage in that yield is substantially low. However, it is widely recognized that the passive alignment method is a proper method needed for reducing the manufacturing costs of the optical coupling module. Thus, the passive alignment method is now applied to 155-Mbps class modules.
When the optical fiber is aligned with the optical circuit, in the optical coupling module fabricated by the above conventional passive alignment method, the coupling efficiency thereof depends on surface roughness and dimension precision of an etched groove in which the optical fiber is disposed, and mutual alignment accuracy between the etched groove and a pattern of the optical circuit as well as the dimension error of the optical fiber itself.
FIGS. 1 and 2a to 2c show the configuration of a conventional optical coupling module, in which FIG. 1 is a perspective view thereof, FIG. 2a is a plan view thereof, FIG. 2b is a side view thereof, and FIG. 2c is a plan view of mask patterns which are aligned with each other. This optical coupling module establishes optical coupling between an optical fiber of an optical communication module and a polymer optical circuit. On a silicon substrate 1, there is formed a V- or U-shaped etched groove 2, in which the optical fiber is disposed. The polymer optical circuit 5 is fabricated such that a core layer 6 of an optical waveguide can be oppositely aligned with a core 4 of the optical fiber. In order to increase the coupling efficiency, a portion through which light can be introduced may be fabricated in the optical waveguide, if desired. In the prior art shown in FIG. 1, a groove 7 is fabricated with a saw-cut method.
Thus, when such an optical bench is completed, a cladding 3 of the optical fiber with a jacket stripped off is fixed in the etched groove 2 with an adhesive.
In order to increase the optical coupling efficiency, the core 4 of the optical fiber fixed in the etched groove 2 formed on the silicon substrate 1 and the core layer 6 of the optical waveguide of the polymer optical circuit should be straightly aligned with each other, as shown in FIGS. 2a and 2b. FIG. 2c is a plan view of a mask having one mask pattern 2B for forming the etched groove on the silicon substrate 1 and the other mask pattern 6B for forming the core layer 6 of the optical waveguide, which are aligned with each other. In order to increase the optical coupling efficiency of the optical coupling module, optical axes of the optical fiber and optical waveguide should coincide with each other. As the optical widths and shapes of the patterns in the respective circuits become more similar to each other, the optical coupling efficiency is increased. When the optical fiber 3 is disposed in the V-shaped etched groove 2, vertical and horizontal positions of the core 4 of the optical fiber depends on the width and size of the etched groove 2, which are determined according to the mask pattern 2B.
The conventional optical coupling module has the following problems: That is, since the mask pattern 6B should be aligned with the etched groove 2 after the etched groove 2 has been first formed on the silicon substrate, or the etched groove 2 should be aligned with the mask pattern 6B after the mask pattern 6B has been first formed on the mask, alignment processes should be carried out at least two times. Thus, this makes fabricating processes complex. Further, since the etched groove for disposing the optical fiber and the core layer of the optical waveguide are separately fabricated, errors in alignment thereof in a predetermined horizontal and direction angle, exposure errors due to steps formed in the etched groove, and the like may be generated. Further, any errors due to temperature variation during the fabricating process may be generated. That is, in a case where the patterns are spaced apart from each other by about 10 cm in contact alignment, an error of about 0.93 μm/° C. is added, considering thermal expansion of a quartz mask. Furthermore, since the problems associated with the flatness of photoresist and the focus depth caused by the steps in the etched groove are serious in a case of a stepper type, the fabricating process becomes difficult or complicated. Thus, there is a problem in that the alignment error of 1 to 2 μm tends to be generated in the mask layers between the core of the optical fiber and the core layer of the waveguide. Even though the V-shaped groove is formed after the optical waveguide has been formed on the silicon substrate, these problems cannot be eliminated.
That is, since the process of forming the V-shaped groove for aligning the optical fiber therein and the process of fabricating and aligning the optical waveguide with the optical fiber have been separately carried out, there is a problem in that certain alignment errors between the core of the optical fiber and the core layer of the optical waveguide should be generated.